1. Field of the Invention
The present invention relates in general to the telecommunications field and, in particular, to a relay station and method for enabling reliable digital communications to occur between two nodes in a wireless relay based network.
2. Description of Related Art
Manufacturers and operators of wireless relay based networks are constantly trying to develop new ways to improve the reliability of digital communications transmitted between two nodes. The traditional wireless relay based networks described below each include one station (node A) that transmits information in coded and modulated digital communications to another station (node B) via one or more relay stations (RSs). The relay station can be part of a base station (BS), a mobile station (MS) or a stand-alone relay station. The nodes A and B can be a BS, a MS and/or a relay station. And, the digital communications could be either uplink communications (link from MS to BS), downlink communications (link from BS to MS), MS to MS communications or BS to BS communications. Examples of different traditional wireless relay based networks and some of their respective drawbacks are briefly discussed below with respect to FIGS. 1-5.
Referring to FIG. 1 (PRIOR ART), there is shown a block diagram of a traditional wireless relay based network 100. The wireless relay based network 100 includes a first node 102 (node A) that transmits information in coded and modulated digital communications to a second node 104 (node B) via one relay station 106. As shown, the first node 102 transmits a channel coded and modulated signal S0(t) over a first link 108 that has a complex channel coefficient h0 such that S0(t) is received at relay station 106 as R1(t)=h0*S0(t)+n1(t), where n1(t) is a noise sequence. The relay station 106 is a repeater that generate a signal S1(t) which is an amplified version of the received sequence and is shown as S1(t)=A*R1(t). The relay station 106 then transmits the generated signal S1(t) over a second link 110 that has a complex channel coefficient h1 such that S1(t) is received at the second node 104 as R2(t)=h1*S1(t)+n2(t), where n2(t) is a noise sequence. There are several drawbacks associated with this type of wireless relay based network 100. First, the amplification of R1(t) at relay station 106 amplifies not only the signal R1(t) but also the noise n1(t) caused by the first link 108. Secondly, the relay station 106 is not very efficient when it amplifies R1(t) since there is a large amount of redundancy in S0(t) due to channel coding.
Referring to FIG. 2 (PRIOR ART), there is shown a block diagram of another traditional wireless relay based network 200. The wireless relay based network 200 includes a first node 202 (node A) that transmits information in coded and modulated digital communications to a second node 204 (node B) via one relay station 206. As shown, the first node 202 transmits a channel coded and modulated signal S0(t) over a first link 208 that has a complex channel coefficient h0 such that S0(t) is received at relay station 206 as R1(t)=h0*S0(t)+n1(t), where n1(t) is a noise sequence. The relay station 206 then decodes, re-encodes, re-modulates and transmits a signal S1(t)=A1*S0est(t) over a second link 210 that has a complex channel coefficient h1 which is received at the second node 204. This is a good solution whenever the relay station 206 makes a correct decision when it estimates and re-encodes S0(t). However, when the relay station 206 makes an incorrect decision, the second link 210 further propagates the information errors and increases the bit error rate of the signal S1(t) transmitted to second node 204.
Referring to FIG. 3 (PRIOR ART), there is shown a block diagram of yet another traditional wireless relay based network 300. The wireless relay based network 300 includes a first node 302 (node A) that transmits information in coded and modulated digital communications to a second node 304 (node B) via one relay station 306. As shown, the first node 302 transmits a channel coded and modulated signal S0(t) over a first link 308 that has a complex channel coefficient h0 such that S0(t) is received at relay station 306 as R1(t)=h0*S0(t)+n1(t), where n1(t) is a noise sequence. The relay station 306 then decodes, re-encodes, re-modulates, amplifies and transmits a signal S1(t) over a second link 310 that has a complex channel coefficient h1 which is received at the second node 304. In this example, the relay station 306 checks the correctness of the decoding of S0(t) using for example a cyclic redundancy check (CRC) and only re-generates, re-modulates and transmits S1(t)=A1*S0(t) in case of correctness, otherwise the relay station 306 simply amplifies and retransmits the received signal as S1(t)=A2*R1(t). This solution is problematic since the relay station 306 needs to make a hard decision on the information symbols in S1(t) instead of having the second node 304 make that hard decision. It is well known that in a communication chain it is advantageous not to make hard decisions until late as possible in the chain. As described below in detail there are some known ways to avoid making hard decisions in relay stations which include using a second relay path or using an automatic repeat request (ARQ) protocol.
Referring to FIG. 4 (PRIOR ART), there is shown a block diagram of yet another traditional wireless relay based network 400. The wireless relay based network 400 includes a first node 402 (node A) that transmits information in coded and modulated digital communications to a second node 404 (node B) via one relay station 406. As shown, the first node 402 transmits a channel coded and modulated signal S0(t) over a first link 408 that has a complex channel coefficient h0 to relay station 406. The relay station 406 then transmits S1(t)=Q(R1(t)) which is a quantized base band representation of R1(t) over a second link 410 that has a complex channel coefficient h1 to the second node 404. In this solution, the second link 410 generally has a larger bandwidth and uses a different air interface protocol than the first link 408. This solution is problematic in that the quantized base band signal S1(t) contains a large amount of data about phase and noise that does not necessarily need to be sent to the second node 404.
Referring to FIG. 5 (PRIOR ART), there is shown a block diagram of a traditional wireless relay based network 500. The wireless relay based network 500 includes a first node 502 (node A) that transmits information in coded and modulated digital communications to a second node 504 (node B) via two relay stations 506a and 506b. As shown, the first node 502 transmits a channel coded and modulated signal S0(t) over a link 508a that has a complex channel coefficient h0 to relay station 506a. At the same time, the first node 502 also transmits S0(t) over a link 508b that has a complex channel coefficient h′0 to relay station 506b. The relay stations 506a and 506b know or can estimate their respective channel coefficients h0, h′0, h1 and h′1 on links 508a,508b, 510a and 510b. As such, relay station 506a can transmit S1(t)=A*conj(h0)*conj(h1)*R1(t) over link 510a to the second node 504. And, relay station 506b can transmit S′1(t)=A*conj(h′0)*conj(h′1)*R′1(t) over link 510b to the second node 504. As is well known the signals R1(t) and R′1(t) because of the scaling and phase shifts involved can be added coherently in a maximum ratio combining sense when they are received at the second node 504. This scheme can be extended to three or more relay paths. Even with multiple relay paths there is still a reliability problem since each relay station 506a and 506b may make a different decision in decoding signals R1(t) and R′1(t) which makes it difficult for the second node 504 to coherently combine signals S1(t) and S′1(t). One possible solution when there are multiple relay paths is for each relay station 506a and 506b to check an error detecting outer code in signals R1(t) and R′1(t). If either R1(t) and/or R′1(t) checks out OK then the respective relay station 506a and/or 506b transmits the corresponding S1(t) and/or S′1(t), otherwise the respective relay station 506a and/or 506b does not send anything or they can send an amplified version of the respective received signal(s). The problem with this solution is that it relies on the following assumptions that (1) at least one relay station 506a or 506b has correctly decoded signal R1(t) and R′1(t) and (2) that the corresponding second link 510a or 510b for such relay station 506a or 506b is sufficiently good to enable the decoding of signals S1(t) or S′1(t). Accordingly, there is a need for a new signal processing solution that can be implemented in a relay station which addresses the aforementioned shortcomings and other shortcomings of the traditional wireless relay based networks 100, 200, 300, 400 and 500. This need and other needs are satisfied by the wireless relay based network and relay station of the present invention.